[Laser] Light Communications Questions. (Stan)

[C. Turner]

Hi Stan,
>
> 1. Looks like power LEDs are advancing nicely, with respect to actual 
> light power output (lumens).
> Any thoughts as to the applicability to light communications ?
>   

The "Bluehaze" page previously mentioned is not well-maintained and can 
be very slow when it is online - and it hasn't been updated for a long 
time, as the host (Tony Sanderson) died several years ago.  Most of the 
optical-communications related content has been aggregated to this page:

http://www.modulatedlight.org/Modulated_Light_DX/MODULATED_LIGHT_DX.html  
- This page includes much of the light-communications content (slightly 
updated) of the Bluehaze page, plus a few other things as can be seen 
from the "root" page, http://modulatedlight.org .

> I just got in a 5W ( 3 x AAA batteries) flashlight and it looks like the 
> single LED is fed DC power
> directly from the battery bank. No driver module. No idea how many 
> lumens it puts out.
>   

I've noticed that most inexpensive "high-power" flashlights treat their 
LEDs rather brutally, relying solely on the internal resistance of the 
batteries supplying them to limit the current.  For example, many 
"3-cell" white LED flashlights use a lot of cheap, white LEDs in 
parallel, connected directly across the battery - only occasionally 
throwing a resistor in series.  With fresh batteries, it's common to see 
2-3 times the rated current of the LEDs being drawn, dropping 
precipitously as the cells go through their normal discharge cycle - 
often ending with the LEDs being significantly under-driven before they 
are mostly discharged.  Such overdriving greatly shortens the "output 
life" of the LEDs - but since one typically doesn't use the flashlight 
for many hours at a time and the fact that even significant degradation 
would likely be unnoticed by the casual user as time passes, little is 
said of such abuse.  A notable exception to this trend are the "Mag 
Light" LED modules:  These are actually switching regulators that seem 
to provide some amount of regulation of the LED's drive with varying 
battery voltages.

> Colored filters (band pass filters) are not commonly available for it yet.
>   

I take it that you are considering filtering the use of white LEDs to a 
specific color?  Doing so will throw away most of the light - especially 
considering that a "white" LED is really a "blue LED plus yellow 
phosphor".  If one wanted red or green, for instance, there's relatively 
little energy at those wavelengths, anyway, and the best result would be 
a rather dim, unsaturated color.  Also do not forget that with white 
LEDs and phosphors, modulation rates are significantly affected by the 
persistence of the yellow phosphor, even to the point of affecting 
speech response, not to mention high-speed data or video.

Choosing the LED specifically for the desired color is a far better option!

> Ebay also has listing for a 12W, single LED, flashlight, using 2 each 
> CR123A ( 3.7V @ 1000 mAh)  batteries.
>   
> 2. Any idea what the band pass of the Kodak red wratten sheet filter 
> material is ?
>  Yes red, but 627 or 635 nM ?  or 600 or 7000 nM,  half power bandwidth ?
>   

Another source of filters are theatrical gels:  The Rosco "Fire Red" (I 
believe that that's the right brand and name together) has a response 
favorable to "red LED" use and these are obtainable in large sheets at 
theatrical supply houses for <$10 - and "swatch" books containing dozens 
of small "swatches" along with their response graphs are often available 
for free, or only a few bucks.

Again, if you are planning to selectively filter a white LED, consider 
that this method is going to have very serious difficulties, as 
mentioned above.  For the filters, they are recommended mostly for 
purposes of providing a modicum of "bandpass" to a radiometric receiver 
system - that it, a system that will respond in some way to any signal 
presented to it.  For use with LEDs, narrowband dielectric filters of 
the sort designed for lasers aren't the best choice!

> 3. Any know issues with modulating the higher power LED sources ?
> Can they handle 10 MHz modulation or is there severely limited
> frequency response due to the actual (large) die construction ?
>   

For non-phosphor LEDs, the capacitance is one of the major factors.  I 
have modulated amplitude-modulated video (including chroma) using a 
single-ended modulator (see:  
http://www.modulatedlight.com/optical_comms/LED_AM_video_link.html  )  
but as mentioned on that same web page, this is *NOT* the way to go 
about it:  I just did this because it was quick and easy to do - and I 
wanted to see if I could get modulation at those sorts of frequencies.

Were video the goal, a better way to do this would be use FM and simply 
turn the LED on and off and there are ways to do this that "force" the 
LED to turn off faster - an important feature considering that a large 
LED like that has thousands of pF of self-capacitance.  For further 
reading on the subject, perusal of the Ronja project (Google it!) can 
provide some insight, although there are some techniques that are even 
better for this.

Detecting 10 MHz energy from a weak optical source whilst maintaining 
good sensitivity is likely the greater challenge.  To do this, APD-based 
detectors or other high-speed devices (such as a hacked Sony "Laser 
Link" receiver) are in order, or a photomultiplier-based detector, along 
with significant optics:  The Hanover group in Germany has a site that 
covers such things with some detail here:   
http://home.arcor.de/W_Fritz/Laser/

> I have seen current sourcing "dimmable"  drivers, but I expect the 
> frequency to be in the low audio range.
>   

Most "dimmable" flashlights that I've seen use PWM techniques.  In one 
case, I have one that the pulsing can be seen if one simply waves the 
flashlight around - although it looks continuous if stationary - the 
rate probably being in the 100's of Hz at most.

For modulation, you might be thinking of PWM - the same sort of scheme 
used by the Ramsey kit, and a scheme that I've also used on several 
occasions - notably on one on the aforementioned web page, and a newer, 
simpler one that I'm in the middle of writing up but have not yet posted 
details or pictures.

Again, the modulatedlight page has several examples of both linear and 
PWM modulation methods.
> 4. I am looking for a source of larger acrylic fresnel lenses.
> At one time this reflector pointed me to a Asian source, but I have lost 
> the reference.
>
>   
You are probably thinking of http://3dlens.com   For "non-image" uses 
(e.g. what is being discussed here) the "coarser" pitch lenses are 
preferable for a number of reasons - namely efficiency and reduced 
scattering:  They are arguably less-fragile, too.

Do *avoid* overhead projector lenses as they are worthless in this 
respect, but do not overlook "page magnifier" lenses, either, which are 
often available at office supply and book stores.  For example, look at 
this device:

http://www.modulatedlight.com/optical_comms/Optical_enclosure_cheap_version.html

This uses a pair of cheap, vinyl (floppy) Fresnel lenses sandwiched 
between picture-frame glass plates and was perfectly capable of spanning 
over 107 miles.  More recently, rigid optical-grade acrylic lenses have 
shown up:  For a recent demonstration to some U of A folks at Tucson, we 
assembled a similar transceiver using these rigid lenses (which were 
bought for $4 each from a University bookstore) which, while cruder than 
the "better" lenses from 3dlens.com (which are somewhat inferior to the 
high-quality surplus lenses that we'd gotten surplus) appeared to be 
better than the flexible vinyl lenses - and these lenses had absolutely 
no difficulty spanning the 7 mile path using a Radio Shack "high power" 
red LED (P/N:  276-020) modulated with my "new" PWM PIC (not yet online) 
with 10's of dB of extra margin - with the light at appearing at the far 
end as the brightest among other city lights.  (Being red made it easy 
to spot, too!)

> I am still in the thinking stage.
>
>   

Thinking is good!


73,

Clint
KA7OEI